1. Field of the Invention
The present invention relates to heat-developable light-sensitive materials. More particularly, it relates to heat-developable light-sensitive materials having high sensitivity and less heat fog, i.e., undesirable blackening at the non-image areas caused by heat-developing.
2. Description of the Prior Art:
The photographic process using silver halides has been most widely practiced hitherto, because excellent photographic properties such as good sensitivity and gradation can be obtained in this process as compared with other photographic processes such as an electrophotographic process or a diazo photographic process. However, silver halide light-sensitive materials used in this process are subjected to development using a developer after image exposure and then subjected to processings such as stopping, fixation, water washing or stabilization to prevent the developed images from fading or discoloring under normal room illumination and to prevent the undeveloped areas (hereinafter called "background") from blackening. Accordingly, these processings take much time and are laborsome. Further, there are problems in that the handling of the chemicals used is dangerous to the human body or the hands and clothes of the workers and the processing room are stained at processing. In addition, the processing solutions discharged into rivers and streams cause environmental pollution. Thus, it is very desired to improve the photographic process using silver halides so that the processings can be carried out in a dry manner without using solution processings and the processed images can be stabilized and so that the background is not colored under normal room illumination.
Therefore, many approaches to achieve such have been attempted hitherto. For instance, a silver halide emulsion containing a 3-pyrazolidone developing agent which can be developed by heating is described in German Patent 1,123,203 and 1,174,157. Further, in this case, the reaction is accelerated when a substance which generates water on heating is present as described in German Patent 1,175,075. Further, German Patent 1,003,578 discloses a silver halide with a fixing agent therefor. However, it is impossible using these techniques to completely stabilize the silver halide remaining in the photosensitive material against light after processing in a dry state. That is, no description relative to a fixation of the material using dry processing is present in the three patents described immediately above, and also in the last patent described immediately above it can easily be understood that putting the material to practical use would be difficult, since an undesired reaction will occur during storage due to the co-presence of the developing agent (reducing agent) and the fixing agent. These defects have now been confirmed experimentally.
Today, the most successful photosensitive material capable of forming a photographic image suitable for processing in a dry state is a heat-developable photosensitive material which utilizes a composition containing a silver salt of an organic acid, a small amount of a silver halide and a reducing agent as the requisite components as described in U.S. Pat. Nos. 3,152,904 and 3,457,075. In these heat-developable light-sensitive materials, after development the silver halide remaining in the light-sensitive material is not stabilized to light but rather allowed to be colored by light. Even so, the same effect as that obtained by stabilization can be attained, since the amount of the silver halide used is small and the major part of the composition comprises stable white or lightly colored organic silver salts which are not blackened by light. Even if a small amount of the silver halide is colored by light, the overall appearance of the material is white or slightly colored, and the slight coloring is not a problem when viewed visually. These light-sensitive materials are stable at room temperature (e.g., about 20.degree.-30.degree. C), but when the material is heated to a temperature of about 80.degree. C, preferably 100.degree. C, after exposure, the organic silver salt oxidizing agent and the reducing agent in the light-sensitive layer undergo an oxidation-reduction reaction due to the catalytic action of the exposed silver halide present in the vicinity and silver is produced. Thus, the exposed areas of the light-sensitive layer are rapidly blackened and a difference in contrast with respect to the non-exposed areas (background) occurs to form images.
However, in these heat-developable light-sensitive materials, there is a tendency toward the production of undesirable blackening (hereinafter designated "heat fog") in the background, and this has become an important problem to be solved. That is, the heat fog relatively reduces the optical density of the image portion formed at the exposed area, and makes the image difficult to distinguish. As a technique of inhibiting heat fog, it is known to add mercury (II) ions to the light-sensitive layer as described in Japanese Patent Publication No. 11113/72. However, mercury compounds are toxic, and a problem of secondary pollution occurs in the regeneration of the waste light-sensitive paper and also a problem of volatilization of the mercury compounds when the material is heat-developed occurs.
Further, many approaches toward decreasing heat fog have been attempted hitherto. However, it is well known to one skilled in the art that it is very difficult to inhibit heat fog without degrading the photographic properties. Sensitivity is, in general, reduced by adding a heat fog inhibiting agent.